During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be achieved by employing high and intermediate aspect ratio tabular grain emulsions.
An emulsion is generally understood to be a "tabular grain emulsion" when tabular grains having an aspect ratio of at least 2 account for at least 50 percent of total grain projected area. The aspect ratio of a tabular grain is the ratio of its equivalent circular diameter (ECD) to its thickness (t). The equivalent circular diameter of a grain is the diameter of a circle having an area equal to the projected area of the grain. An emulsion is understood to be a "high aspect ratio tabular grain emulsion" when tabular grains having a thickness of less than 0.3 .mu.m have an average aspect ratio of greater than 8. The term "intermediate aspect ratio emulsion" is employed when, through tabular grain thickening above 0.3 .mu.m and/or low grain mean ECD, an average aspect ratio in the range of from 5-8 is exhibited. Generally, tabular grain emulsions exhibit average tabular grain aspect ratios of at least 2. The term "thin tabular grain" is generally understood to be a tabular grain having a thickness of less than 0.2 .mu.m. The term "ultrathin tabular grain" is generally understood to be a tabular grain having a thickness of 0.06 .mu.m or less. The term "high chloride" refers to grains that contain at least 50 mole percent chloride based on silver. In referring to grains of mixed halide content, the halides are named in order of increasing molar concentrations--e.g., silver iodochloride contains a higher molar concentration of chloride than iodide.
The overwhelming majority of high and intermediate aspect ratio tabular grain emulsions contain tabular grains that are irregular octahedral grains. Regular octahedral grains contain eight identical crystal faces, each lying in a different {111} crystallographic plane. Tabular irregular octahedra contain two or more parallel twin planes that separate two major grain faces lying in {111} crystallographic planes. The {111} major faces of the tabular grains exhibit a threefold symmetry, appearing triangular or hexagonal. It is generally accepted that the tabular shape of the grains is the result of the twin planes producing favored edge sites for silver halide deposition, with the result that the grains grow laterally while increasing little, if any, in thickness after parallel twin plane incorporation.
While high aspect ratio tabular grain emulsions have been advantageously employed in a wide variety of photographic and radiographic applications, the requirement of parallel twin plane formation and {111} crystal faces pose limitations both in emulsion preparation and use. These disadvantages are most in evidence in considering tabular grains containing high chloride concentrations. It is generally recognized that silver chloride grains prefer to form regular cubic grains--that is, grains bounded by six identical {100} crystal faces. Tabular grains bounded by {111} faces in silver chloride emulsions often revert to nontabular forms unless morphologically stabilized.
While high and intermediate aspect ratio tabular grain silver bromide emulsions were known to the art long before the 1980's, Wey U.S. Pat. No. 4,399,215 produced the first tabular grain silver chloride emulsion. The tabular grains were of the twinned type, exhibiting major faces of threefold symmetry lying in {111} crystallographic planes. An ammoniacal double-jet precipitation technique was employed. The thicknesses of the tabular grains were high compared to contemporaneous silver bromide and bromoiodide tabular grain emulsions because the ammonia ripening agent thickened the tabular grains. To achieve ammonia ripening it was also necessary to precipitate the emulsions at a relatively high pH, which is known to produce elevated minimum densities (fog) in high chloride emulsions. Further, to avoid degrading the tabular grain geometries sought both bromide and iodide ions were excluded from the tabular grains early in their formation.
Wey et al U.S. Pat. No. 4,414,306 developed a twinning process for preparing silver chlorobromide emulsions containing up to 40 mole percent chloride based on total silver. This process of preparation has not been successfully extended to high chloride emulsions. The highest average aspect ratio reported in the Examples was 11.
Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) developed a strategy for preparing a high chloride emulsion containing tabular grains with parallel twin planes and {111} major crystal faces with the significant advantage of tolerating significant internal inclusions of the other halides. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with a grain growth modifier having as its function to promote the formation of {111} crystal faces. Adsorbed aminoazaindenes, preferably adenine, and iodide ions were disclosed to be useful grain growth modifiers. Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II), significantly advanced the state of the art by preparing high chloride emulsions containing tabular grains with parallel twin planes and {111} major crystal faces using an aminoazaindene growth modifier and a gelatino-peptizer containing up to 30 micromoles per gram of methionine. Since the methionine content of a gelatino-peptizer, if objectionably high, can be readily reduced by treatment with a strong oxidizing agent (or alkylating agent, King et al U.S. Pat. 4,942,120), Maskasky II placed within reach of the art high chloride tabular grain emulsions with significant bromide and iodide ion inclusions prepared starting with conventional and universally available peptizers.
Maskasky I and II have stimulated further investigations of grain growth modifiers capable of preparing high chloride emulsions of similar tabular grain content. Tufano et al U.S. Pat. No. 4,804,621 employed di(hydroamino)azines as grain growth modifiers; Takada et al U.S. Pat. No. 4,783,398 employed heterocycles containing a divalent sulfur ring atom; Nishikawa et al U.S. Pat. No. 4,952,491 employed spectral sensitizing dyes and divalent sulfur atom containing heterocycles and acyclic compounds; and Ishiguro et al U.S. Pat. No. 4,983,508 employed organic bis-quaternary amine salts.
Bogg U.S. Pat. No. 4,063,951 reported the first tabular grain emulsions in which the tabular grains had parallel {100} major crystal faces. The tabular grains of Bogg exhibited square or rectangular major faces, thus lacking the threefold symmetry of conventional tabular grain {111} major crystal faces. In the sole example Bogg employed an ammoniacal ripening process for preparing silver bromoiodide tabular grains having aspect ratios ranging from 4:1 to 1:1. The average aspect ratio of the emulsion was reported to be 2, with the highest aspect ratio grain (grain A in FIG. 3) being only 4. Bogg states that the emulsions can contain no more than 1 percent iodide and demonstrates only a 99.5% bromide 0.5% iodide emulsion. Attempts to prepare tabular grain emulsions by the procedures of Bogg have been unsuccessful.
Mignot U.S. Pat. No. 4,386,156 represents an improvement over Bogg in that the disadvantages of ammoniacal ripening were avoided in preparing a silver bromide emulsion containing tabular grains with square and rectangular major faces. Mignot specifically requires ripening in the absence of silver halide ripening agents other than bromide ion (e.g., thiocyanate, thioether or ammonia).
Endo and Okaji, "An Empirical Rule to Modify the Habit of Silver Chloride to form Tabular Grains in an Emulsion", The Journal of Photographic Science, Vol. 36, pp. 182-188, 1988, discloses silver chloride emulsions prepared in the presence of a thiocyanate ripening agent. Emulsion preparations by the procedures disclosed has produced emulsions containing a few tabular grains within a general grain population exhibiting mixed {111} and {100} faces.
Mumaw and Haugh, "Silver Halide Precipitation Coalescence Processes", Journal of Imaging Science, Vol. 30, No. 5, Sept./Oct. 1986, pp. 198-299, is essentially cumulative with Endo and Okaji, with section IV-B being particularly pertinent.
Symposium: Torino 1963, Photographic Science, Edited by C. Semerano and U. Mazzucato, Focal Press, pp. 52-55, discloses the ripening of a cubic grain silver chloride emulsion for several hours at 77.degree. C. During ripening tabular grains emerged and the original cubic grains were depleted by Ostwald ripening. As demonstrated by the comparative Example below, after 3 hours of ripening tabular grains account for only a small fraction of the total grain projected area, and only a small fraction of the tabular grains were less than 0.3 .mu.m in thickness. In further investigations going beyond the actual teachings provided, extended ripening eliminated many of the smaller cubic grains, but also degraded many of the tabular grains to thicker forms.
Japanese published patent application (Kokai) 02/024,643, laid open Jan. 26, 1990, was cited in a Patent Cooperation Treaty search report as being pertinent to the subject matter claimed, but is in Applicants' view unrelated. The claim is directed to a negative working emulsion containing a hydrazide derivative and tabular grains with an equivalent circular diameter of 0.6 to 0.2 .mu.m. Only conventional tabular grain preparations are disclosed and only silver bromide and bromoiodide emulsions are exemplified.
In the precipitation of silver halide emulsions it is the most common practice to perform the entire precipitation reaction in a single reaction vessel. Nevertheless, so-called "dual-zone" precipitations have also been reported. In dual-zone arrangements silver and halide ions are brought together to form grain nuclei in a first area and then transported to a second area for grain growth. For many years emulsion was recirculated from the second (growth) area to the first (nucleation) area, but more recently arrangements have been reported that do not recirculate any portion of the emulsion from the second (growth) area to the first (nucleation) area, thereby completely isolating grain nucleation from grain growth. Specific illustrations of dual-zone precipitation are provided by Mignot U.S. Pat. No. 4,334,012, Urabe U.S. Pat. No. 4,879,208, and European published patent applications 326,852, 326,853, 355,535, 370,116, 368,275 and 374,954.
Although it was known for many years that the performance of silver halide emulsions can be modified by the introduction of transition metal ions during grain precipitation, it was generally assumed that the counterion of the transition metal ion, except when it happened to be halide ion, did not enter the grain structure and that the counterion selection was unrelated to photographic performance. Janusonis et al U.S. Pat. No. 4,933,272; McDugle et al U.S. Pat. Nos. 4,933,272, 4,981,781, and 5,037,732; and Keevert et al U.S. Pat. No. 4,945,035 were the first to demonstrate that ligands capable of forming coordination complexes with transition metal ions are capable of entering the grain crystal lattice structure and producing modifications of photographic performance that are not realized by incorporation of the transition metal ion alone. Thereafter, by hindsight, it was realized that earlier disclosures of adding transition metal ion dopants, either as simple salts or as coordination complexes, had inadvertently disclosed useful ligand incorporations. Of these inadvertent teachings, the incorporation of iron hexacyanide during grain precipitation is the most notable and is illustrated by Shiba et al U.S. Pat. No. 3,790,390; Ohkubo et al U.S. Pat. No. 3.890.154; Iwaosa et al U.S. Pat. No. 3,901,711 and Habu et al U.S. Pat. No. 4,173,483,
Evans et al U.S. Pat. No. 5,024,931 discloses a photographic silver halide emulsion comprised of radiation sensitive silver halide grains exhibiting a face centered cubic crystal lattice structure containing on average, at least one pair of metal ions chosen from group VIII, periods 5 and 6, at adjacent cation sites of the crystal lattice. Increased speed and reduced low intensity reciprocity failure are demonstrated in silver bromide emulsions.
Silver halide emulsions having high chloride contents, e.g., greater than 50 mole percent chloride based on silver, are known to be very desirable in image-forming systems due to the high solubility of silver chloride, which permits short processing times and provides less environmentally polluting effluents. Nevertheless, the higher the chloride content of a silver halide emulsion, the more difficult it is to achieve high and stable radiation sensitivity (sometimes referred to in the photographic art as "speed"). One reason for this is that conventional emulsions having high chloride contents exhibit a severe propensity to deterioration upon aging or storage. As a consequence, such an aged or stored emulsion when processed, produces a higher minimum density than the "fresh" emulsion. This increase in minimum density, commonly referred to as "fog", is attributable to the formation of a low level of reduced silver formation that occurs independently of imagewise exposure. In color photography, fog is typically observed as image dye density, rather than directly as silver density. Changes in fog and sensitivity are particularly troublesome in color photographic elements that comprise multiple color layers since such changes can vary from layer to layer which results in a color imbalance and reduction in quality.
Materials known in the photographic art as photographic "stabilizers", as distinguished from the general class of "antifoggants", have been used in the past to protect radiation sensitive silver halide emulsions against changes in sensitivity and fog upon aging and storage. One skilled in the art readily recognizes the distinction between the use of photographic stabilizers, which combat fog and sensitivity changes that occur upon storage, and those materials, categorized as antifoggants, which combat fog caused by such things as the inherent nature of the radiation sensitive silver halide emulsion (which may produce chemical fog) or the conditions of development of the emulsion, for example, development for protracted periods of time or at temperatures above normal. A more detailed discussion of this distinction between antifoggants and stabilizers can be found in G. F. Duffin, Photographic Emulsion Chemistry, Chapter 7, The Focal Press, London and New York (1966), Research Disclosure, August 1976, Item 14851 and U.S. Pat. No. No. 2,728,663. Research Disclosure is published by Kenneth Mason Publication, Ltd., the Old Harbourmaster's, 8 North Street, Emsworth, Hampshire P010 7 DD, England.
An extensive description of photographic stabilizers and antifoggants which are indicated to be useful for avoiding instability that increases minimum density in negative-type emulsion coatings (i.e., fog) or that increases minimum density or decreases maximum density in direct-positive emulsion coatings is set forth in Research Disclosure, Vol. 308, December 1989, Item 308119, Section VI. In addition, Nishikawa et al , U.S. Pat. No. No. 4,960,689, describes a color photographic material that comprises a silver halide emulsion containing at least 50 mole percent chloride and a compound that is referred to as an antifoggant. The antifoggant described contains a thiosulfonate group. The patent alleges that the color photographic material has high sensitivity and small reciprocity failure and storage fog. Also, Japanese published Patent Application (Kokai) 03/208,041, laid open Sep. 11, 1991, describes a silver halide color photographic material having a silver halide emulsion layer in which the emulsion is prepared in the presence of a thiosulfonate compound. The application alleges that the color material undergoes less fogging during emulsion coating, storage of the emulsion and high speed development.